A metal molded article has been widely replaced with a disk-shape resin molded article including injection molding articles such as a synthetic resin gear, a pulley or the like. These articles are conventionally molded with an engineering plastic by an injection molding technology. The disk-shape resin molded article usually comprises a rim forming a peripheral area, a boss forming a concentric inner site, and a web having a disk configuration for connecting the rim and the boss, and is molded unitarily as a monobloc article by the injection molding. These molded articles are intensely required to have severe precise roundness or circularity of the peripheral or inner surface.
However, for example, in the case of a synthetic resin gear (or toothed wheel) having gear teeth at the outer region of the rim, the strain of the molded gear is increased with widening the gear teeth due to cooling-contraction or shrinkage of the synthetic resin. Thus, the injection molding for forming the gear deteriorates dimensional precision or accuracy of the gear teeth. In order to overcome these disadvantages, it is intended to reduce or thin the thickness of both sides of the web for suppressing the contraction of the teeth in the cooling-solidification process. This intention unfortunately causes the reduction in rigidity of the molded article, and therefore these articles are unsuitable for applications required to have a high mechanical strength.
Thus, it is desired that the rigidity of the articles is retained by thickening the web as well as reducing the contraction ratio of thicken resin articles to inhibit the deformation of the products. For realization of these characteristics, an injection-compression molding method is proposed for inhibiting the contraction or shrinkage of the web and the formation of shrinkage cavities. This method comprises a step for injecting a molten resin into a cavity for forming a thicken web, and a step for pressing the thicken web in a thickness direction by advancing a movable mold member from one side to the other side of the web in an injection metal mold.
As shown in FIG. 10, a resin-molded gear 1 comprises a cylindrical boss 2, a web 3 extending outwardly from the boss and having an uniform thickness, a circular rim 4 formed outwardly on the web, and a gear tooth 5 formed on the rim 4. When a compression force F acts on the one side face A of the web 3 in the injection-compression molding process, the edge of the boss 2 is pulled or attracted outwardly as shown in an arrow direction to deform outwardly the inner surface B of the boss. Therefore, the inner configuration or dimension of the boss 2 may be warped. Further, as shown in FIG. 11, the compression force F on the one side face A of the web 3 causes a force pulling or attracting the edge of the rim 4 inwardly as shown in an arrow direction to deform inwardly the outer circumferential surface C of the rim. Thus, the outer configuration or dimension may also be warped. In more detail, as shown in FIG. 12, the compressing force F on the surface A acts the boss 2 and/or the rim 4 via the uniform thickness web 3, and the unbalanced local strain affects concentrically on the axially middle site 2a of the boss 2 or the axially middle site 4a of the rim 4 (see arrow direction) to cause deterioration of the dimensional accuracy. That is, the axially center site 2a of the boss is deformed inwardly, the axially center site 4a of the rim is outwardly deformed, and these deformations deteriorate the configuration or dimensional accuracy of the boss 2 and the rim 4 as well as the gear tooth 5. To solve these disadvantages, the shape or dimension of the boss and rim is designed with considering the deformation amount. However, these modifications complicate the processing or working of the metal mold including a shape of an undercut and require forcible release of molded articles from the metal mold to deteriorate the dimensional precision of the molded articles.